A plurality of catalysts for purifying exhaust gas are conventionally arranged in series upstream of an exhaust path with a high exhaust gas temperature of a vehicle engine, such as a diesel engine or a gasoline engine.
It is known that, at this time, one or more of the catalysts, which is/are located upstream in the flow of exhaust gas, is/are placed lateral to the other(s), which is/are located downstream in the flow of exhaust gas, to reduce the area for the catalysts (see, e.g., Patent Document 1).
Patent Document 1 discloses disposing a first exhaust gas treatment unit substantially perpendicular to a second exhaust gas treatment unit in a housing so that a side surface of the second exhaust gas treatment unit overlaps at least 50% of the upstream part of the first exhaust gas treatment unit.